In the field of optical communications, there has been used for a long time the simplest system in which intensity modulation is performed as signal modulation and, as demodulation, light intensity is directly converted to an electric signal using an optical detector. However, in recent years, in order to comply with the high bit rate exceeding 40 Gbps, the system in which phase modulation is performed as signal modulation has been attracting attention. There are two methods for demodulating a phase modulation signal. One is a method (coherent optical fiber communication) in which signal-modulated and transmitted light is demodulated by allowing it to interfere with light from a local oscillator provided on the receiver side. The other is a method (differential phase shift keying system) in which a signal-modulated light is split into two. Then, they are multiplexed by shifting the timing by one bit of signal of signal modulation to interfere with each other, and then the shift of the phase is converted to the light intensity signal to be demodulated. Of these two methods, being different from the case of the coherent optical fiber communication, in the differential phase shift keying system, a frequency of the signal light and a frequency of the local oscillator do not have to be synchronized. Since it is comparatively easy to implement the differential phase shift keying system, it has been attracting attention as a system which is nearing practical use. Depending on the number of phases to be modulated, this system is called either a differential binary phase shift keying (DBPSK or DPSK) or a differential quadrature phase shift keying (DQPSK).
With reference to FIG. 1, the modulation method in differential phase shift keying will be explained. Light 101 on which differential phase shift keying is applied enters a delay line interferometer 102. Then, the modulated light 101 is split into two by a light splitting component such as a half beam splitter 103. One of the split beams of light is given an optical path length of one bit (about 7.5 mm, for example, when the signal modulation frequency is 40 GHz) with respect to the other split beam of light by a delaying unit 104 comprising mirrors and is set so that the optical path length difference of the split beams of light becomes a value made by multiplying a wavelength of the light by an integer (that is, the phase difference is zero). Then, the two split beams of light are again multiplexed by the half beam splitter 105, and two interference beams of light 106 and 107 are generated. In this regard, as focusing on the interference light 106, the interference is constructive when the phase shift amount between the adjacent bits is 0 and the interference is destructive when the amount of phase shift is π. As a result, the interference light 106 is converted to the one with intensity of the interference light corresponding to the amount of phase shift between the adjacent bits. The interference light 107 is in a state where its phase differs from that of the interference light 106 by π. Therefore, it is destructive when the interference light 106 is constructive and it is constructive when the interference light 107 is destructive, resulting in the output whose intensity of light is reversed. By detecting the intensity difference of these beams of interference light with use of a balanced optical detector 108 and a differential detector 110 comprising a trans-impedance amplifier 109, a demodulated signal is obtained.
As shown in FIG. 2, demodulation in DQPSK is performed by using two delay line interferometers similar to the ones used in demodulation of differential phase shift keying. To be specific, a modulated beam of light 200 on which differential phase shift keying is applied is split into two by a half beam splitter 201. Then, the split beams of light are led to respective delay line interferometers 202 and 203. Further, interference beams of light generated in the respective delay line interferometers are detected by differential detectors 204 and 205. With respect to the delay line interferometer 202, a delaying unit 206 is set so that the optical path length difference of the two split beams of light becomes a value made by multiplying the wavelength by an integer. However, in the case of the delay line interferometer 203, a delaying unit 207 is set so that the optical path lengths of the two split beams of light differ from each other by (n+¼)λ (where n is an integer and λ is a wavelength of light). In this regard, when the amount of phase shift between adjacent bits is 0 or π, there occurs constructive interference or destructive interference in the delay line interferometer 202. On the other hand, in the case of π/2 or 3π/2, there occurs constructive interference or destructive interference in the delay line interferometer 203. Therefore, it becomes possible to demodulate a DQPSK-modulated signal from the output of the differential detectors 204 and 205. Furthermore, it is possible to demodulate a differential phase shift keying signal of a given M value with use of the same configuration.
For implementing the above delay line interferometer, there are two embodiments possible. One is an embodiment in which a light waveguide is mainly used. The other is an embodiment in which a free space optical system with use of a bulk optical component is used. While mass production is easy in the former case, it also has demerits such as requiring temperature control, consuming a lot of electric power, and being large in size. On the other hand, in the latter case, power consumption can be kept low, being compactly structured. Thus, it is attracting attention as a promising mode of implementation.
Generally, in regard to the modulated light in the optical communications, when arriving at the demodulator, polarization is disturbed by anisotropy of the material of the optical fiber through which the light has passed, and is in a random polarization state. For this reason, the characteristic of the modulator needs to be independent of the polarization of the demodulated light. PDFS (Polarization Dependent Frequency Shift) is a phenomenon in which modulated light is demodulated according to the polarization state, as if it has a different frequency (wavelength), causing degradation of the signal quality. Therefore, when implementing the demodulator, reduction of PDFS is a problem to be addressed.
PDFS is caused when an optical path length difference between the two split beams of light (or a phase difference) has polarization dependence in the delay line interferometer inside the demodulator. Above all, in the free space optical system, a main factor responsible for the above polarization dependence is the imperfection of the half beam splitter to be used for splitting and multiplexing beams of light. To be exact, it is a principal factor that p polarized light and s polarized light to the light splitting surface of the half beam splitter undergo different phase changes when passing through and being reflected on the surface.
JP-A-2008-224313 (corresponding to official gazette No. US2008/0218836) is intended to solve the problem of PDFS by disposing, on the optical path of the split light, a phase compensation component for offsetting the above phase shift. In this case, relative phases of the p polarization and s polarization are varied according to the above phase compensation component, which negates the relative phase difference occurring in the half beam splitter and eliminates the polarization dependency.
Also, JP-A-2008-241950 (corresponding to official gazette No. US2009/0027683) is intended to solve the problem of PDFS by splitting and multiplexing beams of light with use of the same half beam splitter and, further, by arranging positions of the two half beam splitter surfaces to be reversed with respect to the split light. In this case, relative phases of reversed orientation are generated when splitting and multiplexing beams of light and, by cancelling both of them, the polarization dependence is eliminated.